Wild Trout Symposium XI—Looking Back and Moving Forward (2014)
Coldwater as a Climate Shield to Preserve Native
Trout Through the 21st Century
Daniel J. Isaak, Michael K. Young1, David Nagel, and Dona Horan2
1
U.S. Forest Service, Rocky Mountain Research Station, 800 E. Beckwith Avenue, Missoula, Montana 59801, USA
2
U.S. Forest Service, Rocky Mountain Research Station, 322 E. Front Street, Suite 401, Boise, Idaho 83702, USA
Abstract—Native trout are culturally and ecologically important, but also likely to undergo
widespread declines as the coldwater environments they require continue to shrink in association
with global warming. Much can be done to preserve these fish but efficient planning and targeting
of conservations resources has been hindered by a lack of broad-scale datasets and precise
information about which streams are most likely to support native trout populations later this
century. Using accurate stream temperature climate scenarios developed in the NorWeST
project, we identify stream habitats for native Cutthroat Trout Oncorhynchus clarkii and Bull Trout
Salvelinus confluentus across northern Idaho and northwestern Montana that are cold enough
to serve as climate refugia and resist invasions by nonnative trout. Climate-safe coldwater
habitats for Cutthroat Trout in the historical scenario encompassed 7,547 – 16,821 stream
kilometers (depending on the local co-occurrence of Brook Trout Salvelinus fontinalis) and 12,189
kilometers for Bull Trout. The majority of coldwater habitats (77%-88%) currently occur on federal
lands, a pattern that will become even more pronounced late in the century if the projected
63%-82% declines in coldwater habitats occur. The information developed for this project,
and accompanying geospatial databases, are also available for a much larger area across
the northwest U.S. to assist managers in strategic decision making about where to allocate
conservation resources to best preserve native trout.
Introduction
From a societal perspective, the marquis
freshwater fish in cold waters across the globe are
salmonines—trout, salmon, and char in the subfamily
Salmoninae. Not only do these fish have commercial,
recreational, and cultural importance, they serve an
array of ecological roles as predators, prey, hosts of
freshwater mussels, and conduits of nutrients from
oceans, lakes, and rivers to headwater tributaries and
their associated riparian habitats. These fish evolved
in and colonized waters throughout the Northern
Hemisphere, but have also been widely introduced
outside their native ranges in suitable waters of the
Southern Hemisphere. Nevertheless, within their
native ranges, every taxon of these fish has undergone
declines over the last two centuries, coincident with
our exploitation of them for food, their habitats for
water extraction or development, and their watersheds
for resources (e.g., Montgomery 2003; Williams et al.
2011). In North America, many taxa or conservation
units within them have been designated as in need
of conservation action e.g., listing under the U.S.
Endangered Species Act, Canada’s Species At Risk
110—Session 3: Struggling with Invasive Species
Act, or state or provincial programs identifying species
of concern. For some taxa, these declines have been
arrested, but restoration to their former habitats has
been difficult and costly, in many cases because
invasive species, including salmonids introduced
outside their historical ranges, now occupy those
habitats (Fausch et al. 2009).
The relatively rapid and pervasive changes in
global climate and stream temperatures (Webb and
Nobilis 2007; Kaushal et al. 2010; Isaak et al. 2012)
in recent decades constitute a further threat to the
persistence of many salmonid populations. Growing
evidence documents shifts in populations of these
fishes (Comte et al. 2013; Eby et al. 2014) as they
attempt to track the distribution of the cold waters on
which they depend. In many cases, these changes are
likely to constrain populations to smaller and more
fragmented headwater habitats (Rieman et al. 2007;
Wenger et al. 2011b). Given limited resources to
conserve fishes that have already undergone broadscale range reductions, further work that addresses
the threat of climate change demands strategic
planning. To that end, managers have begun to ask
Wild Trout Symposium XI—Looking Back and Moving Forward (2014)
which locations are likely to retain thermally suitable
habitats of adequate size and connectivity for native
salmonids despite anticipated changes in climate. If
such climate refugia could be identified, it would allay
fears of species losses this century and the refugia
could serve as cornerstones in the development of
strategic conservation networks. Moreover, because
growth and survival of nonnative fishes are precluded
in exceptionally cold streams where native salmonids
often thrive, refugia with temperatures below certain
thresholds would be resistant to invasions and require
limited management intervention. In effect, cold water
could be used as a “climate shield” to protect native
salmonids against climate change and invasive species
this century.
Accurately modeling the distribution of coldwater
stream habitats is now possible because of the
availability of nationally consistent stream geospatial
data (Cooter et al. 2010; Wang et al. 2011), highresolution climate scenarios for stream temperature
and flow (Isaak et al. 2010; Wenger et al. 2010; Isaak
et al. 2011), and new statistical models for stream data
that enable development of unbiased information from
large databases and accurate predictions of patterns
throughout stream networks (Ver Hoef et al. 2006;
Isaak et al. 2014; Ver Hoef et al. 2014). Our goal is to
demonstrate how these data and tools may be used to
identify current and future distributions of coldwater
habitats for two species of native salmonid fishes—
Bull Trout Salvelinus confluentus and Cutthroat Trout
Oncorhynchus clarkii—across selected river basins in
the upper Columbia River basin in Idaho and Montana.
The work described herein is the initial phase of
delineating specific climate refugia for these species
across a much broader area of the northwestern U.S.
Methods
The study area included northern Idaho (north of
the Salmon River basin) and northwestern Montana
within the Columbia River basin (Figure 1). Stream
elevations ranged from 200 to 2,500 m between the
Continental Divide to the east and the mouths of major
rivers to the west.
To delineate the fish-bearing stream network,
geospatial data for the NHDPlus 1:100,000-scale
stream hydrography layer (Cooter et al. 2010) were
downloaded from the Horizons Systems website
(http://www.horizon-systems.com/NHDPlus/index.
php) and filtered by minimum flow and stream
slope. Each reach in the NHDPlus hydrography layer
already has many descriptive attributes calculated,
among them stream slope (Wang et al. 2011). Stream
reaches with slopes exceeding 10% were trimmed
from the network because fish densities are low in
these reaches, steep reaches are prone to post-fire
debris torrents that can extirpate salmonid populations
(Brown et al. 2001), and because they occur at the top
of the network where slopes become progressively
steeper. Summer streamflow values were downloaded
from the Western US Flow Metrics website (http://
www.fs.fed.us/rm/boise/AWAE/projects/modeled_
stream_flow_metrics.shtml; Wenger et al. 2010) and
linked to each reach in the hydrography layer through
the COMID field. Summer flow values for three
climate periods were available from that website: a
Figure 1. Stream temperature observations (n = 9,969) used to fit the NorWeST model in the study area (panel a)
and an interpolated map of August mean temperatures representing the 1980s historical period (panel b).
Session 3: Struggling with Invasive Species—111
Wild Trout Symposium XI—Looking Back and Moving Forward (2014)
historical period (1970–1999, hereafter referred to as
1980s) and two future periods (2030–2059, hereafter
2040s; 2070–2099, hereafter 2080s) associated with
the A1B climate trajectory. Peterson et al. (2013b)
described the relationship between summer flows
and stream width and found that summer flows of
0.034 m3s (1.2 ft3s) approximated stream widths of
1.5 m. Trout presence in streams narrower than 1.5 m
becomes sporadic due to small habitat sizes (Peterson
et al. 2013a), so the network was also trimmed to
exclude reaches with summer flows < 0.034 m3s.
Application of the slope and flow criteria reduced the
original set of blue-lines in the NHDPlus hydrography
layer from 84,191 stream km to 35,850 km, the latter
of which was used to represent fish habitat in the
baseline 1980s period.
Summer stream temperature scenarios represented
by August means were downloaded from the NorWeST
website (www.fs.fed.us/rm/boise/AWAE/projects/
NorWeST.html; Isaak et al. 2011) and used to attribute
the baseline hydrography layer. The number of stream
temperature observations used to fit the NorWeST
model in the study area was 9,969 (Figure 1a) and
the model had good predictive accuracy across these
observed sites over a wide range of historical climate
variation (r2 = 0.92; RMSE = 0.78°C). NorWeST
scenarios were available for the same A1B climate
trajectory and future climate periods described above at
a 1-km resolution for all streams in the study area.
Thermal niches for Bull Trout and Cutthroat
Trout encompass colder temperatures than do those of
nonnative salmonids such as Brook Trout Salvelinus
fontinalis, Brown Trout Salmo trutta, and Rainbow
Trout O. mykiss (Wenger et al. 2011a,b). Thus,
important spawning and juvenile rearing habitats for
allopatric populations of the native trout species are
often upstream of the distribution of nonnative trout.
Temperatures in Bull Trout natal habitats are so cold
that overlap with nonnative trout is limited (Rieman
et al. 2006; Isaak et al. 2010). However, Cutthroat
Trout spawn over a wider temperature range and
displacement by nonnative salmonids is common
where species overlap in warmer streams. To estimate
temperatures that delineated suitable natal Bull Trout
habitats and buffered Cutthroat Trout populations
against invasions, we referenced stream locations
where juvenile trout of either species (<150 mm) had
been sampled against mean August water temperature
calculated using the NorWeST S1 historical scenario
112—Session 3: Struggling with Invasive Species
(which represented the climate composite from
1993-2011) at the same location. For Bull Trout, the
juvenile survey data came from longitudinal surveys
of 74 streams across the interior Columbia River basin
(Rieman et al. 2007). The mean stream temperature
at the farthest downstream locations of juvenile Bull
Trout in those streams was 10.9°C (99% CI, 10.7–
11.1°C), so we used ≤ 11°C to delineate natal Bull
Trout habitats.
Locations of juvenile Cutthroat Trout in the study
area were obtained from 863 reach surveys (Young
et al. 2013; M. K. Young, unpublished data). Those
data indicated juvenile Cutthroat Trout occurred
most frequently in stream reaches with temperatures
less than 10°C, but juveniles were not uncommon
where August mean temperatures approached 14°C.
Brown Trout and Rainbow Trout become more
common in warmer streams (Wenger et al. 2011a),
but those surveys did not include enough sites to
reliably estimate temperatures at the upstream limits
of these species. For that, we relied on information
from a regional fish survey database that included
approximately 20,000 site surveys (S. Wenger,
unpublished data). Cross-referencing those surveys
with the NorWeST S1 historical scenario indicated
that Rainbow Trout and Brown Trout rarely occurred
where temperatures were < 12°C so this value was
used as one criterion for delineating Cutthroat Trout
natal habitats. Another nonnative species, Brook Trout,
has a colder thermal niche than Rainbow Trout or
Brown Trout. The regional fish survey database and
earlier research (Al-Chokhachy et al. 2013) indicate
that Brook Trout are most common in reaches with
mean August temperatures near 12°C and become
relatively rare where temperatures are < 10°C. Hence,
we used ≤ 10°C to delineate Cutthroat Trout habitats
that would resist Brook Trout invasions in streams
where this nonnative also occurred.
To determine the amount of stream habitat that
met the above criteria, we queried the trimmed
hydrography layer to identify those reaches ≤ 10, 11,
and 12°C that also had summer flows > 0.034 m3s.
The query was done for the historical and two future
periods and the total length of coldwater streams
summarized. Results from that query were crossreferenced with land ownership compiled for the
ICBEMP project (Quigley and Arbelbide 1997) to
determine the administrative status of coldwater refuge
streams.
Wild Trout Symposium XI—Looking Back and Moving Forward (2014)
Results
accounted for most of the projected reductions
(94–98%) in coldwater habitat length. The large
majority of coldwater refugia streams in the historical
period (77–88%) were on federal lands, and this will
increase in the future because most non-federal lands
are at lower elevations where streams are relatively
warm. Approximately 23% of the historical coldwater
habitats are considered protected based on special
land designations (18.7% in Forest Service Wilderness
Areas, 2.6% in Glacier National Park).
Considerable thermal heterogeneity existed across
the study area due to the complex topography (Figure
1). Portions of the stream network with significant
amounts of cold water occurred along the Continental
Divide to the east and at high elevations scattered
throughout various mountain ranges. August stream
temperatures in the historical period ranged from 5.1°C
to 24°C and averaged 12.5°C. Average temperatures
were projected to increase by 1.4°C in the 2040s and
2.5°C in the 2080s. Application of the 12°C thermal
criteria to the historical temperature scenario suggested
that Cutthroat Trout had 16,821 km of streams cold
enough to impede invasions by Brown Trout and
Rainbow Trout (Table 1; Figure 2), but only 7,547 km
if the more restrictive thermal criteria of 10°C was
applied to limit Brook Trout invasions. Coldwater Bull
Trout habitats were intermediate between these two
extremes at 12,189 km.
Relative to the historical baseline, the amount of
habitat in the 2080s that was ≤ 10°C was predicted
to decline by 82% to 1,355 km, whereas habitat ≤
12°C was predicted to decline by 63% to 6,169 km.
Future habitat reductions reflected both summer
flow declines that truncated headwater streams and
summer temperature increases that shifted isotherms
upstream. Of these two effects, temperature increases
Discussion
Consistent with many previous assessments
(e.g., Rieman et al. 2007; Wenger et al. 2011b),
our results indicate that coldwater habitats for
salmonids will markedly decline as a consequence of
climate change this century. Unlike many previous
studies, however, our approach uses accurate stream
temperature model scenarios and species-specific
thermal criteria developed from large biological and
temperature databases to greatly increase the precision
of our projections. The approach is conservative in that
it assumes nonnative species will invade all thermally
suitable habitats and restrict the distribution of both
native species. That is clearly not the case at present;
Brook Trout, for example, are absent from large
numbers of basins they could seemingly occupy
Table 1. Kilometers (% in parentheses) of stream habitat by land administrative status for Bull Trout and Cutthroat
Trout that are cold enough to resist invasion by other trout species during historical and future periods.
1980s
Land status1
<10°C
<11°C
2080s
<12°C
<10°C
<11°C
<12°C
Private
691 (9.2)
1,655 (13.6)
3,200 (19.0)
82 (6.1)
217 (6.7)
556 (9.0)
Tribal
115 (1.5)
202 (1.7)
290 (1.7)
33 (2.4)
60 (1.9)
100 (1.6)
State/City
133 (1.8)
243 (2.0)
375 (2.2)
13 (1.0)
49 (1.5)
127 (2.1)
COE
2 (0.1)
2 (0.1)
2 (0.1)
0 (0.0)
2 (0.1)
2 (0.1)
BLM
59 (0.8)
111 (0.9)
149 (0.9)
0 (0.0)
7 (0.2)
34 (0.6)
FWS
0 (0.0)
0 (0.0)
4 (0.1)
0 (0.0)
0 (0.0)
0 (0.0)
NPS
149 (2.0)
321 (2.6)
456 (2.7)
21 (1.6)
65 (2.0)
136 (2.2)
FS-wilderness
1,674 (22.2)
2,274 (18.7)
2,688 (16.0)
452 (33.3)
871 (27.0)
1,329 (21.5)
FS-nonwilderness
4,725 (62.6)
7,380 (60.5)
9,657 (57.4)
754 (55.6)
1,948 (60.5)
3,885 (63.0)
Total
7,547
3,219
6,169
12,189
16,821
1,355
1
Abbreviations: COE, Corps of Engineers; BLM, Bureau of Land Management; FWS, Fish and Wildlife Service; NPS, National Park Service;
FS, Forest Service.
Session 3: Struggling with Invasive Species—113
Wild Trout Symposium XI—Looking Back and Moving Forward (2014)
Figure 2. Streams for Bull Trout (panels a and c) and Cutthroat Trout (panels b and d) that are cold enough to
resist invasion by other trout species during historical and future periods.
(Al-Chokhachy et al. 2013; M. K. Young, unpublished
data; Wenger et al. 2011a). Nevertheless, the future
spread of Brook Trout, Brown Trout, and Rainbow
Trout—either by natural colonization or humanassisted (and generally illegal) transport—seems likely
(Rahel 2004), and the coldwater streams highlighted
here can serve as climate-safe and invasion-resistant
refuge habitats.
Despite seemingly inevitable future declines, the
long-term persistence of Bull Trout and Cutthroat
Trout in the study area does not appear to be in
jeopardy. There are thousands of stream kilometers
that are cold enough to provide suitable habitats even
with substantial future climate change and warming
this century. Most of these coldwater habitats occur
on federal lands at higher elevations, particularly the
National Forests. Future climate change will only
114—Session 3: Struggling with Invasive Species
enhance this pattern, emphasizing the role that federal
land management can play in maintaining a climate
shield to conserve native coldwater species. Many
coldwater refuge streams already occur in designated
wilderness areas and support disproportionate numbers
of strong Cutthroat Trout and Bull Trout populations
of (e.g., Rieman and Apperson 1989; Kershner et al.
1997), but wilderness designation may be insufficient
insurance against climate change. For example, many
portions of the Selway-Bitterroot Wilderness Area
in Montana and Idaho are expected to warm beyond
the thresholds acceptable to Bull Trout, and to favor
more thermally tolerant trout species at the expense of
Cutthroat Trout. Such areas will constitute a dilemma
for biologists wishing to actively manage watersheds
to retain coldwater species. However, coldwater
habitats in adjacent non-wilderness public and private
Wild Trout Symposium XI—Looking Back and Moving Forward (2014)
lands have fewer restrictions and might be strategically
targeted for conservation actions that bolstered native
trout populations.
Identification of coldwater streams is only the
beginning of climate-smart native trout conservation.
The next steps in this process include developing
demographically based estimates of habitat sizes
needed for population persistence, implementing the
approach across entire species ranges and large river
basins, and providing climate refugia information
in geospatial digital map formats for easy use with
numerous native trout conservation initiatives, such
as those sponsored by the multi-agency Western
Native Trout Initiative. The approach taken is also
generalizable in that it could be extended to other
native headwater species that are dependent on cold
water (e.g., Rocky Mountain Tailed Frog Ascaphus
montanus or Coastal Giant Salamander Dicamptodon
tenebrosus). In the northwestern U.S., doing so
simply requires developing an estimate of habitat size
needed for population persistence and species-specific
thermal criteria, both of which can be derived using
broadly available geospatial stream data and biological
survey information. Coldwater climate refugia
could also be delineated in other parts of the U.S. or
globally where native organisms persist and thrive in
cold environments that constrain nonnative species
invasions. The primary limitation for identifying such
areas currently is the limited availability of stream
temperature data (and perhaps ecological data for
poorly surveyed species), but monitoring networks and
databases have begun to grow rapidly in recent years
with the advent of inexpensive sensors and reliable
protocols for data collection (Isaak et al. 2011; Isaak
et al. 2013). In all cases, better information about the
locations and likely persistence of coldwater climate
refugia will contribute to more strategic allocation
of limited conservation resources, help rally support
among multiple stakeholders concerned about the
future of coldwater fauna, and increase the odds of
long-term species preservation.
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